In a known hydraulic pressure control valve of this kind responding to load, a pair of pistons cooperates with a pair of valves to control the braking hydraulic pressure transmitted to the wheel cylinders, the pistons being mounted in a single housing in a parallel relation. One end of each piston protrudes from the housing in a liquid-tight manner so as to be axially slidable. The pistons are displaced toward their other ends by an actuating mechanism in response to the load carried by the vehicle.
The performance of this prior art hydraulic pressure control valve may vary from product to product or become unstable according to the structure of the aforementioned actuating mechanism. The hydraulic pressure control valve mentioned above is disclosed in, for example, Japanese Utility Model Laid-open Nos. 40060/1981 and 59655/1983 and U.S. Pat. No. 4,332,423.